1. Field of the Invention
The present invention relates to a drawing false-twisting method capable of producing a textured bulk yarn through the continuous series of processes of twisting a yarn, heat-setting the twisted yarn and untwisting the heat-set yarn, and a drawing false-twister for carrying out the drawing false-twisting method. More particularly, the present invention relates to a drawing false-twisting method and a drawing false-twister capable of reducing irregularities in the crimps of the textured bulk yarn to prevent irregularities, such as thin bands, in a woven fabric.
2. Related Art Statement
The construction of a drawing false-twister for carrying out a drawing false-twisting method will be described with reference to FIG. 7 showing one of the drawing false-twisting units of the drawing false-twister for drawing and false-twisting polyester multifilament yarns or nylon multifilament yarns, which, in general, are called POYs (partially oriented yarns). A multifilament yarn 2 unwound from a yarn package 1 is guided by guides 3, 4 and 5 to a first feed roller 6. Then the yarn 2 fed upward into a primary heater 7 disposed in an upright position by the feed roller 6 reaches a guide 8 disposed above the primary heater 7. Since the primary heater 7 is relatively tall, the yarn 2 travels obliquely downward over a service passage 19 from the guide 8 toward a guide 11. The yarn is cooled by a cooling plate 9 disposed between the guide 8 and 11 and is false-twisted by a belt-type false-twisting device 10. The false-twisted yarn is guided by the guide 11 to a second feed roller 12. The respective surface speeds of the first feed roller 6 and the second feed roller 12 are determined so that the surface speed ratio between the first feed roller 6 and the second feed roller 12 is equal to a predetermined draw ratio. A portion of the yarn extending before the false-twisting device 10 is twisted and a portion of the yarn extending behind the false-twisting device 10 is untwisted. The twisted portion of the yarn is drawn at the predetermined draw ratio and is heat-set by the primary heater 7. Then, the yarn is overfed into a secondary heater 13 by the second feed roller 12 and is guided by a guide 14 to a third feed roller 15. Then, the drawn false-twisted yarn travels via guides 16 and 17 and is wound in a package 18 by a winder. The secondary heater 13 heat-sets the crimps of the overfed drawn false-twisted yarn to reduce the torque of the drawn false-twisted yarn and to retain the bulkiness of the same. The secondary heater 13 is not used if the desired characteristics of the drawn false-twisted yarn do not need secondary heating.
Thus, the drawing false-twisting method draws twisted yarn, heat-sets the drawn false-twisted yarn, cools the drawn false-twisted yarn and untwists the drawn false-twisted yarn to obtain the textured bulk yarn by the series arrangement of the first feed roller 6, the primary heater 7, the cooling plate 9, the false-twisting device 10 and the second feed roller 12 arranged in that order in the direction of travel of the yarn. The drawn false-twisted yarn is cooled by the cooling plate 9 for the following purposes. The yarn Y is heated at a high temperature of 160.degree. C. or above by the primary heater 7. If the yarn Y heated at such a high temperature is twisted by the false-twisting device 10, the false-twisting device 10 is unable to twist the yarn Y stably and the twisting belts of the false-twisting device 10 may be affected by heat. Therefore, the yarn Y is cooled to a low temperature of 70.degree. C. or below by the cooling plate 9 disposed between the primary heater 7 and the false-twisting device 10.
A prior art cooling plate 9 will be described hereinafter with reference to FIGS. 8 and 9. The cooling plate 9 is formed by bending a stainless steel strip in a shape having a V-shaped cross section and a guide groove 20 is formed in the bottom surface of the cooling plate 9. While the yarn Y travels in contact with the surface of the guide groove 20, the yarn Y is cooled mainly through heat exchange between the yarn Y and the cooling plate 9. Since twists inserted in the yarn Y by the false-twisting device 10 propagate, the yarn Y tends to roll in the direction of the twists. For example, the yarn Y tends to roll in the direction of the arrow j (FIG. 9) if Z-twists are inserted in the yarn Y. Therefore, the guide groove 20 is formed in a curved sectional shape so that the rolling force of the yarn Y and the force biasing the yarn Y toward the bottom of the guide groove 20, balance each other. To cope with switching in Z-twist or S-twist, the curved sectional shape is symmetrical, and is generally the guide groove 20 having a curved surface of radius r. The cooling plate 9 is curved in the shape of an arc of a circle having a radius R and ensures the satisfactory contact of the yarn Y with the surface of the guide groove 20. The heat transferred from the yarn Y to the cooling plate 9 is radiated naturally from the outer surface 21 of the cooling plate 9. Therefore, temperature distribution in the cooling plate 9 declines at a gentle gradient from the inlet portion of the cooling plate of a comparatively high temperature toward the outlet portion of the same of a comparatively low temperature.
It is known that periodically light filling bars form in a fabric when the fabric is woven by using dyed yarns produced by dyeing textured bulk yarns produced by drawing and false-twisting specific POYs, namely, POYs having a high oil content, by the prior art drawing false-twister. It was found that these light filling bars are attributable to dyeing irregularities, and dyeing irregularities are attributable to the irregular crimps of the drawn false-twisted yarns. It was also found that the period of variation of irregular crimps coincides with the period of appearance of peaks of tension during untwisting at a point A on the drawing false-twister of FIG. 7.